This invention relates to an amplifier circuit comprising a control transistor and an output transistor whose control electrode is coupled to the main current path of the control transistor. The control transistor is understood to mean the transistor or transistor configuration receiving the control signal and the output transistor is understood to mean the transistor or transistor configuration applying the amplified signal to a load.
It is a common phenomenon that when the temperature rises due to increased dissipation, the differential base-emitter input resistance of a transistor may become negative, that is to say, the base-emitter voltage decreases as the base current increases. This phenomenon is known in the literature as forward second breakdown or thermal breakdown and is caused by the strong positive thermal dependence of the collector current. In a power transistor the collector current will be inclined to concentrate in the central part of the transistor leading to higher dissipation in this central part. Due to above-mentioned thermal dependence, a hot spot may then be developed, more specifically, at high voltages, which leads to the destruction of the transistor.
If a plurality of transistors are connected in parallel as regards their d.c. current setting--this is understood to mean that their base-emitter input circuits on the one hand and their collector-emitter output circuits on the other hand are included in parallel branches--the parallel circuit will become unstable and the current will no longer be uniformly distributed over the parallel branches and in the worst case will flow through only a single branch if the temperature at which the differential input resistance becomes negative is reached. This means that it is not readily possible to arrange a plurality of transistors in parallel in order to tolerate a larger total amount of dissipated power. This phenomenon also occurs in power transistors whose internal structure corresponds to a plurality of parallel-arranged transistors. The same holds for power output stages of integrated circuits comprising parallel-arranged transistors or transistor configurations.
A prior art solution to the problem posed is to include resistors in each of the emitter paths of the parallel-arranged transistors. This causes the differential input resistance to become negative only at a higher dissipation level so that the problem is postponed as it were.
Using resistors in the emitter paths of parallel-arranged transistors, however, leads to a reduction of the range in which the various output transistors can be driven to full voltage swing and hence of the attainable output power. Furthermore, dissipated energy in the form of heat is developed in the resistors. Furthermore, this solution becomes less effective as the base-emitter voltage increases and the collector current decreases.
An amplifier circuit which does not include any resistors in the emitter paths but uses thermal couplings for solving the problem posed is described in U.S. Pat. No. 3,952,258. This known amplifier circuit comprises a plurality of parallel branches each including at least a control transistor and an output transistor. The various control and output transistors are spatially arranged in such a way that each output transistor has a stronger thermal coupling to a control transistor different from the control transistor that has an electrocoupling with the relevant output transistor. Ideally, such a cross-coupling leads to equal currents flowing through the various output transistors despite any differences in temperature. In principle, the power generated by each output transistor is equal but, as a result of a difference that may occur if dissipating heat develops in an output transistor, a disturbance of the thermal balance may still occur. Practice has shown that an ideal thermal cross-coupling cannot be realised and that there will always be a slight instability. The wiring of a cross-coupled structure is furthermore rather complicated, more specifically, if the number of output transistors is greater than two.